Add a helper to group the writes to PMU counter, this will be
used to delay setting the event period to pmu::pmu_enable()

Cc: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: NOlof Johansson <olof@lixom.net>
Acked-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NSuzuki K. Poulose <suzuki.poulose@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>